It is now well established that the enamel organ has two major phases of activity: matrix production and maturation. Differences in cell structure between secretory and post-secretory ameloblasts, stratum intermedium and papillary cells, are characteristic of each phase. The post-secretory enamel organ possesses many features that are usually found in transport epithelia. Among these are increased surface area, extensive nexus-type junctions, increased pinocytosis and increases numbers of mitochondria. It has been shown recently that the maturation (post-secretory) ameloblasts undergo cyclic change from ruffle-ended to smooth-ended cells at their interface with the enamel surface. These changes involve extensive membrane reorganization and the remodeling of cell-to-cell contacts of the gap and tight junction variety. In this application the principal investigatator proposes to examine the cell surfaces of the secretory and post-secretory enamel organ by transmission electron microscopy, freeze-fracture methods, cytochemistry and autoradiography. The major focus of the research will be to gain a better understanding of regional membrane specializations within the enamel organ and to correlate the development and/or regression of these specializations with enamel organ function. The primary research objectives will be to study the presence and extent of gap and zonula occlaudens junctions at each phase of enamel organ activity by freeze-fracture techniques, to study the distribution and concentration of intramembrane particles in the various cell types, to localize membrane Na-K-ATPase by both cytochemistry and 3H-ouabain binding, to investigate membrane cycling, including coated vesicle function in the enamel organ, and to study membrane sterol distribution with filipin binding cytochemistry. These studies will be carried out on the enamel organ of continuously developing incisors of rats and mice. Other appropriate models such as the enamel organs of certain fishes will be used to answer specific questions. The proposal sets forth the hypothesis that the post-secretory enamel organ produces local osmotic gradients needed to remove water and solutes from the enamel. Cell membrane structure and function are the key features of the hypothesis. The research objectives are developed in relation to this hypothesis.